1. Field of the Invention
This invention relates to sound delivery systems for biological applications and particularly to a closed sound delivery system with a flat frequency response to 30 KHz.
2. Description of the Prior Art
This invention relates to the accurate generation and monitoring of sound at the eardrum. More specifically, this invention relates to the apparatus for minimizing linear distortions associated with coupling sound from an electro acoustic generator to the eardrum, as well as those associated with coupling sound at the eardrum to an electro acoustic receiver. By means of this invention, a time pattern of sound pressure which accurately resembles the time pattern of voltage applied to the electroacoustic generator is produced at the eardrum. Additionally, the time pattern of the output of electroacoustic receiver accurately resembles the time pattern of sound pressure at the eardrum.
Through investigations of relationships between measurable characteristics of acoustic stimuli and those of electrophysiological and behavioral responses, an increased understanding of the operation of the auditory system is achieved. The determination of these relationships depends upon the ability to both generate and monitor specific sound stimuli at a convenient location such as the eardrum. For example, studies employing speech or speech-like signals in which the time pattern of the acoustic waveform is of particular concern require a closed system for delivering and monitoring acoustic signals at the eardrum possessing the wide bandwidth with smooth and relatively flat response characteristics. Even in studies employing single frequency periodic signals, stimulus generation and specification is aided considerably by the use of such a system.
Considerable knowledge about the operation of the auditory system continues to be obtained from acute physiological experiments on small mammals. Sound systems used in these experiments vary widely in their design but generally contain three elements: (1) an earphone for generating sound; (2) a coupler consisting of air channels through which the sound travels between the earphone and the eardrum, and (3) a probe tube microphone used to monitor sound at the eardrum. Although the band-width of a given system is ultimately limited by that of the earphone used, the relative smoothness of the pressure response at the eardrum depends upon the acoustic properties of the earphone as well as those of the coupler, the ear, and the probe tube monitoring system. Accordingly, consideration of the acoustic properties of each of these elements and how they interact is required if specific performance criteria are to be achieved via a systematic design. Nonetheless, many current designs are based mainly on considerations of mechanical convenience. Consequently, acoustic performance is often marginally adequate and is generally achieved through a trial and error procedure which involves packing the air channels of both the coupler and the probe tube monitor with steel wool or nonabsorbent cotton. The end result of such practice is generally a compromise between sufficient bandwidth and sufficient smoothness of freuency response. It is possible to avoid this compromise by considering the acoustic properties of the individual elements of the system and by basing the design on fundamental principles of lumped-element and transmission line acoustics.
In A. R. Moller, "Acoustic Stimulator For Use in Acute Experiments On Cats", QPR No. 73, Res, Lab. Electron. MIT. (1964) pp. 181-185, there is disclosed a closed sound delivery system. The system includes a sound generator transducer connected through a cavity to a transmission tube and a receiver transducer connected to a transmission tube. Both tubes are filled with steel wool in order to flatten the system frequency response, resulting in altered response properties when either tube fills with blood. The frequency response of the receiver remains flat only to about 800 Hz and exhibits numerous resonant peaks and dips at higher frequencies. The use of a washer to decrease the air volume in front of the receiver transducer is indicated although its effect on high frequency response is not well documented. There is no suggestion of suppressing acoustical energy emanating from the periphery of the sound generator transducer in order to minimize frequency reponse deviations due to the effects of destructive and constructive interference.
In Alan Barnebey, David C. Nagel, and Edward C. Carterette, "An Earphone Coupling System for Acute Physiological Studies," J. Acoust. Soc. Am., Vol. 52, Number 4, Part 2, pp. 1256-1262, 1972, there is disclosed an earphone system using a generally conical configuration to provide a gradual transition from a large diameter transducer to a small diameter transmission tube. Claim is made to a relatively flat frequency response from about 40 Hz to 10 KHz.
In J. P. Barton, J. K. Koester, and M. Mitchner, "Probe-tube Microphone for Pressure-Fluctuation Measurements in Harsh Environments," J. Acoust. Soc. Am., Vol. 62, No. 5, pp. 1312-1314, 1977, it is suggested that an acoustical resistance be placed midway along the length of the transmission tube. A resultant reasonably flat frequency reponse approaching 10 KHz is claimed.
In Leo Beranek, Acoustic Measurements, John Wiley & Sons, New York (1949), pp. 186-189 and 730-735 there appears a discussion of a sound probe developed by R. H. Nichols (OSRD Report 4666, Electro-Acoustic Lab., Harvard University) which shows an attenuation of about 20 dB/octave above 100 Hz.
In W. West, "Probe Microphones as Laboratory Standards," Acoustica, Vol. 4 pp. 131-133, 1954, there is described an arrangement in which the transducer is disposed part way along a terminated transmission path.
R. W. Leonard, "Probe-tube Microphones," J. Acoust. Soc. Am., Vol. 36, No. 10, pp. 1867-1871, 1964, reviews several different probe-tube microphone arrangements. However, the specific configuration in which the tube is terminated at its microphone end by a volume in series with an acoustic resistance is not described.
In W. West, "Probe Microphones as Laboratory Standards," Acoustica Vol. 4, pp. 131-133, 1954, various means of obtaining an approximate broad band resistive termination for probe microphones are described. However, because none of the configurations described provide high impedance at low frequencies they are not suitable for making accurate measurements in small volumes such as that presented by the ear at low frequencies.
David P. Egalf, "Mathematical Modeling of a Probe Tube Microphone," J. Acoust. Soc. Am., Vol. 61, No. 1 pp. 200-205, 1977, discusses techniques for mathematical modeling of probe-tube microphones.
U.S. Pat. No. 3,985,960, "Stereophonic Sound Reproduction With Acoustically Matched Receiver Units Effecting Flat Frequency Response at Listener's Eardrums," Wallace, Jr., discloses a transducer which contacts an outer human ear. The frequency characteristics of the sound generating unit are selected to compensate for the characteristics of the human ear and the transducer itself. Unlike the present invention which relates to closed acoustical systems for biological applications this disclosure relates to entertainment systems and attempts to provide a flat frequency response to the eardrum notwithstanding distortions introduced by the passage of the acoustical signal through the sound generator and through the outer ear to the inner ear. No means of monitoring sound at the eardrum is provided. Additionally, it is not clear whether performance claims are based on real ear measurements or on those obtained from an ear simulator. The constant displacement, high acoustical output impedance, transducer feeds into a cavity which reduces the driving impedance and provide a flat frequency response pressure output. In contrast, the present invention uses a large diameter low output impedance transducer in conjunction with a suppressor element having a central aperture in which the resultant cavity is a byproduct of the geometrys of the suppressor element rather than an impedance reducing element.
Bell Laboratories, Memorandum For File, "A New Kind of Headphone Receiver," R. L. Wallace, Jr., Oct. 10, 1974, provides a disclosure which is similar to that of the patent.
An oral disclosure of the subject matter of this invention was made to the American Acoustical Society on Dec. 13, 1977 and an abstract of the disclosure was provided for review and approval by the Society in August or September 1977 and distributed to Society members on or about Nov. 22, 1977.
The above references are being included in the file history of the application for this patent for ease of access.